In recent years, there have been widely used broad-area semiconductor laser devices having a broad current injection stripe width as light excitation sources of solid-state lasers, fiber lasers, and double-clad erbium-doped optical amplifiers or the like, and further as light sources for direct processing of metal or resin. As the range of their application fields becomes wider as mentioned above, the broad-area semiconductor laser devices are increasingly required to have not only high output, but also high reliability (See Japanese Unexamined Patent Publication JP-A 2001-308445). The broad-area semiconductor laser devices include gain waveguide semiconductor laser devices.
FIG. 4 is a sectional view showing a constitution of a conventional gain waveguide semiconductor laser device 1. In the semiconductor laser device 1, a first cladding layer 3, a first optical waveguide layer 4, an active layer 5, a second optical waveguide layer 6, a second cladding layer 7, a current confinement layer 8, and a contact layer 9 are formed on a surface 2a of a planar semiconductor substrate 2 in a thickness direction Z thereof. A current injection region (which is a current injection stripe) 11 is formed in a midportion in a transverse direction Y of the planar semiconductor substrate 2, which is perpendicular to both an oscillation direction X and the thickness direction Z thereof.
<100>DLD (Dark Line Defect) is known as one reason for deteriorating reliability of the broad-area semiconductor laser devices. Further, it is known that the <100>DLD grows with defects introduced by scribing or the like at the time of manufacturing a semiconductor laser device, as starting points. When a current is injected to a semiconductor laser device, spontaneous emission light having an intensity equal to the laser oscillation threshold or lower is emitted. In the conventional gain waveguide semiconductor laser device 1 as shown in FIG. 4, no light confinement structure is formed in the transverse direction thereof. For this reason, the spontaneous emission light which occurred in the current injection region 11 is absorbed by the active layer 5, while being propagated by the first and second optical waveguide layers 4 and 6 which are part of the laser structure thereof so as to reach a proximity of device ends 12 in the transverse direction Y of the semiconductor laser device 1. The resultant carriers generated according to the aforementioned process, will recombine at defects of the device ends 12 which are introduced by scribing or the like. The defects grow due to the recombination energy, and dislocation develops towards a crystal orientation <100>, resulting in the formation of <100>DLD.
FIGS. 5A and 5B are observation views obtained by observing the <100>DLD which has grown with a defect introduced by scribing in the conventional semiconductor laser device 1 shown in FIG. 4. FIG. 5A is an appearance image of the semiconductor laser device 1, and FIG. 5B is an image obtained by photographing electroluminescence of the semiconductor laser device 1 in FIG. 5A by means of a high sensitive CCD (Charge Coupled Device). When a gain waveguide semiconductor laser device such as the semiconductor laser device 1 shown in FIG. 4 is driven for a long period, due to the defect present in the device ends 12 in the transverse direction, the DLD grows from end facets of the device ends 12 in a 45-degree direction within one hypothetical plane parallel to both the oscillation direction X and the transverse direction Y. That is, the DLD grows along the crystal orientation <100>. The <100>DLD grows rapidly when reaching the current injection region 11, which becomes a serious problem of stopping laser oscillation.